This invention relates to lightwave communications systems, and more particularly to multiwavelength lightwave communications systems.
In single wavelength long distance optical communications systems fiber amplifiers, such as erbium-doped fiber amplifiers, are periodically spaced along the fiber path to compensate for the transmission losses that accumulate as the light traverses the transmission fibers and other optical components along the system. The gain of each amplifier in the cascade can match the signal loss in the portion of the transmission path that follows the previous amplifier in the cascade. It is known (see, e.g., C. R. Giles and E. Desurvire, "Modeling Erbium-Doped Fiber Amplifiers", Journal of Lightwave Technology, Vol. 9, No. 2, pp. 271-283, February 1991) that a cascade of saturated fiber amplifiers acts to self-regulate the signal power through the transmission system. Accordingly, the power output of the first amplifier in a cascade of saturated fiber amplifiers is duplicated at the output of all the subsequent amplifiers along the system.
There is currently considerable interest in building large, multiwavelength communications systems to support the envisioned high-capacity information networks of the future. Such multiwavelength systems will advantageously have increased signal carrying capacity. Furthermore, and even more significantly, the multiple wavelengths will be used for the purposes of signal routing. As in the single-wavelength optical communications systems, in multiwavelength systems cascades of optical amplifiers will be required to compensate for the losses that accumulate as the light traverses the transmission fibers, and the larger losses that the signals encounter from the optical switches and routers along the optical signal path.
The most significant technical obstacle standing in the way of such large, amplified multiwavelength communications systems is the nonuniform gain spectrum of fiber amplifiers. Although amplifier gain spectra are typically flat within three decibels over a bandwidth of approximately 20 nm, these relatively modest gain nonuniformities in a single amplifier will accumulate along a cascade, resulting in exponentially rising interchannel power variations. For example, a 3-dB difference in the gain at one wavelength as compared to the gain at a second wavelength, when accumulated through ten cascaded amplifier stages, results in a power ratio equal to one-thousand. Therefore, the weaker channels in a wavelength-multiplexed system are likely to fall to power and signal-to-noise ratio levels that render them undetectable. Such behavior is in effect fundamental and inherent in the spectroscopy of erbium ions in silica glass, which is the only material system, so far, that has succeeded in providing practical optical gain for lightwave communications systems. It is because of this fundamental property of the erbium ions in the silica glass that the power regulation noted above as being achievable with a cascade of saturated fiber amplifiers in a single wavelength optical system will not be effective in a multiwavelength optical system. Specifically, in a multiwavelength system, the output of each conventional fiber amplifier will be regulated on a total power basis, and not on a channel-by-channel basis.
Various prior art approaches to this problem have been proposed. In a first approach (A. R. Chraplyvy, J. A. Nagel, and R. W. Tkach, "Equalization in Amplified WDM Lightwave Transmission Systems", IEEE Photon. Technol. Lett., Vol. 4, No. 8, pp. 920-922, August 1992) the transmitter power is selectively boosted for wavelengths that propagate weakly through the system. Such an approach may be effective in modest-size point-to-point links, but is not promising in networks, especially those with dynamically reconfiguring signal paths. A second approach (K. Inoue, T. Kominato, and H. Toba, "Tunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers", IEEE Photon. Technol. Lett., Vol. 3, No. 8, pp. 718-720, August 1991) uses fixed filters to selectively suppress wavelengths that propagate too strongly. This approach has also achieved some success, but it is not adjustable in the event of component or amplifier-inversion-level variations. Moreover, it cannot be scaled to very large system sizes due to critical matching difficulties. In a third approach (S. F. Suet al, "Gain Equalization in Multiwavelength Lightwave Systems Using Acoustooptic Tunable Filters", IEEE Photon. Technol. Lett., Vol. 4, No. 3, pp. 269-271, March 1992) channel-suppression filters are embedded in servo-loops, one servo-loop being used per channel per amplifier. Although it is in principle effective, this approach is complex. Furthermore, this thud approach imposes increased system losses that must themselves be compensated for by additional gain stages.
An object of the present invention is to provide in a multiwavelength lightwave communications system power regulation on a channel-by-channel basis with a minimum of optical component complexity.